DWG/DFX plans snapping and modeling
A variation of that should be possible, certainly.

DXF import was a bit sketchy last time I tried. You may need to export in DXF R14 format or something relatively old like that. FBX is more reliable.

You can certainly snap to vertices of the imported file. You'll probably add a dummy mesh (e.g. a plane), enter edit mode, then repeatedly press 'e' to extrude vertices along the background (imported) mesh/lines.

Alternatively you can just select vertices from the import mesh and extrude them. Might want to separate them from the original mesh to suit your workflow. Just press 'p' when you've selected a bunch of vertices that you want to turn into a new object.

Personally I prefer place walls as a single line along the wall centrelines and extrude vertically. Then you can adjust wall thickness easily using a Solidify modifier and you have fewer vertices to contend with, which makes cutting holes for doors and windows easier.